According to the measurement principle on which the thermal mass flowmeter is based, two temperature sensors can be fitted in the media flow, which acts on them, in order to transfer heat to the media flow and to measure its temperature. In this case, resistance thermometers are primarily used as temperature sensors. The flow can be calculated from the heating power and the temperature of a heated resistance thermometer, using generally known mathematical relationships.
DE 199 39 942 A1 from the same applicant discloses a thermal mass flowmeter. This thermal mass flowmeter has a pulsed electrical heating element and a temperature sensor which is thermally operatively connected to the flowing fluid and whose electrical resistance is temperature-dependent. In this case, the sensor element can be heated by means of the heating element. Electronic means are provided in order to determine the flow and to detect the time profile of a heating and cooling process which takes place in the sensor element.
Heating elements and sensor elements are in this case accommodated in a common housing. The thermal operative connection between the sensor element and the medium flowing past is influenced by the choice of material for the housing and of a possible filling material within the housing, as well as the geometric configuration of the housing.
In the above-described thermal mass flowmeter, the housing has a taper in the area of the sensor element, in order to increase the thermal operative connection. In principle, an optimum measurement effect and high measurement accuracy can be achieved by as good a thermal operative connection as possible, that is to say a maximum heat transfer from the heating element to the flowing medium, and by minimum heat dissipation into the sensor connections or holders.
In order to achieve a sensor system with a short response time, the thermal mass of the sensor system and its packaging environment should be kept as small as possible. On the other hand, numerous industrial applications—for example, in the foodstuffs and pharmacology sectors—require metal encapsulation of the sensor system, for example, by means of stainless steel. In addition to hygienic aspects, such metal-encapsulated sensor systems have the advantage of being considerably more resistant to corrosive media, and can therefore be used even in severe environments. However, appliances such as these require complex packaging for the sensor system, since the sensor system and its packaging must be stable over a wide temperature range and numerous different materials are generally used, with different thermal coefficients of expansion.
U.S. Pat. No. 5,880,365 discloses a metal-encapsulated sensor system for a thermal mass flowmeter having a metal cap. The metal cap has a cylindrical shape and is formed by welding a hollow cylinder and a circular cover together. The ratio of the circumference to the cross section of this sensor system is a critical quality parameter. Resistance wires are used internally as sensors and are wound around a likewise cylindrical former, which is in turn surrounded by the cylindrical metal cap. In order to achieve a sensor system with a low thermal resistance, the cylindrical metal cap is in this case plastically deformed into the flowing medium. In addition, the use of a gland contributes to increased thermal insulation of the cylindrical former.
DE 102 16 532 A1 describes a metal-encapsulated sensor system in which a thermal measurement element is mounted on a rectangular ceramic mount. The ceramic mount is in turn surrounded by a cylindrical metal cap, and is fixed therein by means of solder tin. In this case, however, the electrical contact-making points of the measurement element are not covered by the solder tin. Furthermore, the ceramic mount is covered by a temperature-resistant glass layer at the location of the measurement element which is mounted on it. Tin metallization is applied to the opposite surface of the ceramic mount to the measurement element. All of these quite complex measures serve to achieve a sensor system with as low a thermal resistance as possible in the flowing medium, and great thermal insulation at the same time within the mount.
In addition, EP 1 835 267 A2 includes a measure for suppression of the heat flow between the heating resistor and its mount. A second active heating element is used for this purpose, and is positioned between the heating resistor and the mount. This type of active heat flow suppression requires a correspondingly large amount of component complexity, however.